On the Duration of Time-Averaging in St. Croix Chione

One of the most compelling modern applications of paleoecological research focuses on utilizing subfossil assemblages to determine the impact of human activity on shallow marine environments. The extent of anthropogenically-mediated changes, however, cannot be determined without a pre-disturbance ecological baseline against which the altered state may be compared. To overcome this limitation, paleoecologists rely on subfossil assemblages of shelly fauna that accumulate in shallow marine environments to characterize the pre-disturbance community composition and establish a timeline of community change. However, through processes including low sedimentation rates, mixing, and differential taphonomic destruction, most shallow marine subfossil assemblages are time-averaged, meaning that specimens within the same stratigraphic horizon may differ in age by as many as several thousand years. As such, without a high-resolution temporal framework, narrowing down precisely when anthropogenic effects began to manifest in shallow marine environments becomes difficult or impossible. In order to use the subfossil record as an accurate measure of St. Croix’s pre-disturbance baseline, this project made use of amino acid racemization (AAR) dating (coupled with radiocarbon calibration) as an effective means to establish the age distribution of the Chione bivalve subfossils therein. Age distributions gleaned from these analyses indicate downward, but not upward mixing, with both recent and older shells being present in the deepest samples and older shells being absent from the shallower horizons. Absolute ages determined through radiocarbon analysis show clusters of ages primarily around the 1960s and the 500s CE (~1500 years old). By expanding our preliminary dataset to include additional age data, we hope to determine: a) the extent of time-averaging in seagrass beds, b) whether deeper intervals are likely to represent more accurate pre-disturbance baselines, and c) how these environments might have changed through the examined interval.